Some of the groups running ads this election season haven't been required to disclose their donors. But as Election Day draws nearer, some of the rules are changing, making campaign ads a riskier business for those who want to keep donors a secret.

'Electioneering' Vs. 'Express Advocacy' Ads

Electioneering ads don't expressly tell you how to vote. Groups are required to disclose the donors behind these ads 60 days ahead of an election.

Express advocacy ads, on the other hand, explicitly tell viewers how to vote. They're not subject to the same disclosure rules, but can violate IRS rules governing social welfare groups.

An Example Of An 'Electioneering' Ad

This spot from Crossroads GPS is an example of an "electioneering" ad. Instead of telling people how to vote, it recommends telling President Obama to stop spending in order to cut the debt.

Crossroads GPSYouTube

An Example Of An 'Express Advocacy' Ad

This Americans For Prosperity Ad is an example of "express advocacy," telling people how to vote by saying "We must replace President Obama."

Republican Mitt Romney's campaign put up 15 new ads Friday in eight battleground states. In those eight states, the Romney and Obama teams have spent $381 million since May 1, according to an NPR analysis of data compiled by the National Journal.

Republicans have outspent Democrats in five of the eight states, and, overall, they lead by $25 million.

Two pro-Republican groups account for two-thirds of all the GOP spending and more than the Romney campaign itself. One group is Americans for Prosperity, which is backed by billionaires David and Charles Koch. The other, cofounded by strategist Karl Rove, is Crossroads GPS.

Americans for Prosperity and Crossroads GPS are technically social welfare organizations, so they aren't required to disclose their donors. But now that the election is fewer than 60 days away, maintaining secrecy is a little harder.

There's a legal "window" that closes 60 days before the election. Once it's shut, groups are required to disclose the money behind so-called "electioneering" ads. Those are TV and radio ads that talk about candidates but don't expressly tell you how to vote.

The window for the presidential campaigns actually closed 30 days ago, since the conventions are considered to be primary elections. Now, as of Friday, the window is closed for Senate and House races, too.

To avoid disclosure rules, social welfare groups can switch to "express advocacy." These kind of ads explicitly advise viewers how to vote or whom to support, but don't have a disclosure requirement. According to tax code, however, social welfare groups aren't supposed to be political organizations. So if a group does too much express advocacy — generally considered to be more than half a group's activity — it can get into trouble with the IRS.

"There are 10s — if not 100s — of millions of dollars in the gamble," says Greg Colvin, a San Francisco lawyer who specializes in the law for tax-exempt organizations. "They're gonna cross their fingers and hope that they spent less than 51 percent of their money on ads that the IRS will not consider to be political."

Crossroads GPS and Americans for Prosperity didn't respond Friday to queries on how they plan to proceed.